In this proposal, we seek to understand the immune response to enteric pathogens using Citrobacter rodentium the natural rodent pathogen as a model organism for the study of pathogenesis of the clinically significant human EPEC and EHEC pathogens. Like the human strains, C. rodentium attach to the intestinal epithelium, lead to loss (effacement) of microvilli, and the formation of actin-rich pedestals underneath the adherent bacteria, a process leading to attaching and effacing (A/E) lesions. Mice infected with C. rodentium develop colitis that resolves within two to three weeks due to clearance of bacteria and development of a protective humoral and CD4 T cell mediated immune response. The laboratory of W. Ouyang showed that IL- 22 was critical for protection against the infection through the induction of anti-microbial peptides of the Reg III family. IL-23 deficient and IL-23R deficient mice that we have generated are both extremely susceptible to C. rodentium. Thus, the IL-23-Th17 pathway is critical for orchestrating immune responses against this enteric pathogen. One important function of IL-23 is to regulate IL-22 expression. IL-22 has been shown to promote wound healing, proliferation, and anti-apoptotic pathways in intestinal epithelial cells and it also up-regulates anti-microbial peptide and mucus production Although IL-22 was originally described as a cytokine produced only by Th17 cells, it is now recognized that IL-22 can be produced in large quantities by innate cells that belong to a growing family of cells called Innate Lymphoid cells (ILCs). Recently the subset of innate cells hat play the most dominant role in protective immunity against enteric pathogens and that produce the most IL-22 in response to IL-23 during the course of an infection was identified as LTi-like cells that expres the surface markers Thy1+CKit+Sca1 CD4+. However in another study it was found that the innate source of IL-22 during infection was not LTi cell but a subset of NK like cells that express the marker NKP46 and the transcriptional factor Rorgt. However, identifying this subset by using markers such as SCA1 or NKp46 to identify this subset may not be reliable, as these markers can be up-regulated upon activation. Although it is clear the ILcs are an important source of IL-22, our data indicate that during Citrobacter Rodentium infection as early as day 4 four post infection a large amount of IL-22 is produced by T cells that express also IL-23R that we call Th22 cells TCR ab bearing T cells and not by ILCs. To study the interplay between ILCs and T cells during the course of an infection by an enteric pathogen we propose the following aims: Aim1: Identification of the Innate subset that mediate protection to enteric pathogen and Aim2: Role of Th22 versus ILcs cells in protective immunity. Understanding how these pathogenic bacteria induce inflammation and host response is critical to understanding how protective immune response can promote the clearance of enteric pathogens.